Strategies for chemically coupling natural or synthetic molecules to biological surfaces are important tools for drug delivery, tissue repair, and fixation of tissue engineered scaffolds for tissue regeneration. Several methods are capable of attenuating, inhibiting or promoting interactions between tissue surfaces as well as between the cells and extracellular matrix (ECM) proteins that comprise them. Electrostatic interactions have been employed by Elbert et al. in the form of poly-L-lysine-graft-(poly(ethylene glycol) polymers that chemisorb to proteins on tissue surfaces, and this approach was explored as a strategy to minimize postsurgical adhesions. Winblade et al. employed phenylboronic acid modified polymers to provide reversible covalent crosslinks to cis-diols in sugar residues of glycoproteins and polysaccharides. Layer by layer (LbL) assembly of polyelectrolytes has been used to apply polymer coatings onto model biological surfaces, the surface of blood vessels and pancreatic islets.
Specific functional groups found in ECM proteins have been exploited for covalent surface modification strategies. For example, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and N-hydroxysuccinimide (NHS) have been used extensively to couple macromolecules containing carboxylic acids to protein amines. Amine-reactive PEG diisocyanates have been used to modify pancreatic islets in order to provide immunoprotection, to create a barrier to platelet adhesion on damaged arteries, and on preclotted Dacron and other model biological surfaces. Aldehyde modified chondroitin sulfate, which also reacts with tissue amines, has been used as a tissue adhesive in both the cornea and cartilage. Photochemical oxidation of native tyrosine residues in collagen II has been used to improve the integration of photopolymerized hydrogels with cartilage.
In contrast to chemical or photochemical approaches, a general strategy for tissue surface modification is directed to biological enzyme mediated crosslinking reactions. Transglutaminases (TG) are calcium-dependent enzymes that catalyze crosslinking between lysine and glutamine residues to form ε-(γ-glutaminyl)lysine isopeptide bonds. (See, Lorand L and Graham R M. Transglutaminases: Crosslinking enzymes with pleiotropic functions. Nature Reviews Molecular Cell Biology 2003; 4: 140-156.) There is growing interest in the use of transglutaminase enzymes for tissue repair and reconstruction. Calcium-independent microbial TG has been used to develop gelatin hydrogels for biomedical adhesives as well as for in vitro expansion of cells. Factor XIII, the circulatory form of TG, has been used to form fibrin matrices for in vitro and in vivo studies of neurite growth, angiogenesis, and cartilage regeneration. The incorporation of bioactive peptides and proteins into these matrices was achieved by including a Factor XIII reactive peptide domain within the molecule. Synthetic polymers have also been modified with Factor XIII substrate peptides, which were then crosslinked by the enzyme into a hydrogel. (See, Sanborn T J, Messersmith P B, and Barron A E. In situ crosslinking of a biomimetic peptide-PEG hydrogel via thermally triggered activation of factor XIII. Biomaterials 2002; 23: 2703-2710.)
A transglutaminase enzyme found in many connective tissues and often referred to as tissue transglutaminase (tTG) was used to form hydrogels through crosslinking of glutamine modified poly(ethylene glycol) (PEG) polymers and a lysine containing polyaminoacid. Hu et al. subsequently employed rationally designed peptide substrates of tTG to modify PEG polymers to form an adhesive hydrogel. (See, e.g., Hu B and Messersmith P. Rational design of transglutaminase substrate peptides for rapid enzymatic formation of hydrogels. Journal of the American Chemical Society 2003; 125: 14298-14299; and Hu B and Messersmith P. Enzymatically cross-linked hydrogels and their adhesive strength to biosurfaces. Orthodontic and Craniofacial Research 2005; 8: 145-149.) More recently, tTG has also been used to couple biomolecules to insoluble peptide assemblies, and was used to enhance cell adhesion and spreading on collagen matrices and synthetic polymers coated with fibronectin.
Relating more specifically to a particular disease state, over 70 million people in the US are afflicted with the debilitating pain and inflammation of osteoarthritis (OA), caused primarily by the action of the potent catabolic cytokine interleukin 1 (IL-1). The effects of IL-1 include increased production of matrix metalloproteases and other inflammatory cytokines, inhibition of extracellular matrix (ECM) synthesis, and cell death. Glucocorticoids down regulate the transcription and translation of IL-1, and hence are a major treatment strategy for reducing the pain and inflammation of OA. Locally administered via intra-articular injections to maximize their effects at the inflamed joint and to minimize adverse systemic effects, glucocorticoids are insoluble suspensions which precipitate in the joint space. Nevertheless, a significant portion of the drug dose is still cleared from the joint, entering the circulatory system and exerting unwanted systemic effects including a drop in endogenous cortisol levels, temporary disruption in glucose metabolism, and immunosuppression. In addition, precipitated crystals of glucocortiods may potentially induce inflammation.
Several approaches have been proposed to increase the retention of glucocorticoids in the inflamed joint, mostly relying on introduction of drug-laden vehicles into the intra-articular space. Liposomes of glucocorticoid palmitate esters have been shown to be more effective in reducing knee swelling than the free drug alone. Another method to improve glucocorticoid retention in the joint capsule is modification of hyaluronan (HA) with methyl prednisolone, which method was shown to have better antioxidant properties on chondrocytes than when the two compounds were administered individually. Microspheres and nanospheres have also been utilized as local drug delivery vehicles to joints. However, performance can be problematic and dependent on factors such as the molecular weight and type of polymer used to form the spheres, the extent of crosslinking and particle size. However, such strategies to retain glucocorticoids in the joint space also rely on precipitation of the drug vehicle in the joint, which can lead to problems of the sort encountered with current treatments: potential inflammation and/or elimination via the lymphatic system. As such, there remains in the art an on-going search for an alternate approach for delivery and retention—to realize the benefits associated with such therapeutics.